1. Field of the Invention
The invention relates to a hydraulic control system of a vehicle power train.
2. Description of the Related Art
Some of known power trains mounted on vehicles include a belt-type continuously variable transmission, a hydraulic lock-up clutch, and the like. The belt-type continuously variable transmission clamps a belt by hydraulic pressure to transmit power, and changes the belt winding diameters to vary speed ratios. The hydraulic lock-up clutch is provided in a fluid power transmission device that is arranged in a power transmission path between a power source and the belt-type continuously variable transmission.
A hydraulic control system of such a vehicle power train includes a large number of various control valves, electromagnetic valves for controlling the control valves, and the like. For example, the hydraulic control system includes a line pressure control valve, a shift hydraulic pressure control valve, a clamping hydraulic pressure control valve, a lock-up control valve, and the like. The line pressure control valve regulates a line pressure, which is a source pressure (control source pressure) of hydraulic pressures at various portions. The shift hydraulic pressure control valve regulates the line pressure, which is the source pressure, and supplies a drive pulley (primary pulley) of the belt-type continuously variable transmission with a shift hydraulic pressure by which the speed ratio of the belt-type continuously variable transmission is controlled. The clamping hydraulic pressure control valve also regulates the line pressure, which is the source pressure, and supplies a driven pulley (secondary pulley) of the belt-type continuously variable transmission with a clamping hydraulic pressure by which the belt clamping pressure of the belt-type continuously variable transmission is controlled. The lock-up control valve operates when the lock-up clutch is engaged or released. In addition, the hydraulic control system includes electromagnetic valves, such as a linear electromagnetic valve, an on/off electromagnetic valve and a duty-controlled electromagnetic valve.
Japanese Patent Application Publication No. 3-213773 (JP-A-3-213773) and Japanese Patent Application Publication No. 2006-153104 (JP-A-2006-153104) describe a hydraulic control system of a belt-type continuously variable transmission. In addition, JP-A-2006-153104 describes a control that is executed when an electromagnetic valve that controls a shift hydraulic pressure control valve or an electromagnetic valve that controls a clamping hydraulic pressure control valve fails.
Incidentally, in the hydraulic control system, control valves or electromagnetic valves that control the control valves may fail because of a mechanical factor, such as a valve stick, or may fail because of an electrical factor, such as a disconnection or a short-circuit in the electromagnetic valves. However, the hydraulic control system of the existing belt-type continuously variable transmission supplies a shift hydraulic pressure to the drive pulley only from the shift hydraulic pressure control valve. Thus, if the shift hydraulic pressure control valve or the electromagnetic valve that controls the shift hydraulic pressure control valve fails, a shift hydraulic pressure controlling a speed ratio may steeply decreases, causing the belt-type continuously variable transmission to be placed in a rapid deceleration state. Then, in a rapid deceleration state, a belt slip, an overrevolution (overspeed), an axle lock, or the like, may occur.
As measures for avoiding such rapid deceleration in the event of a failure, a control valve, or the like, having a back-up function may be provided for the hydraulic control system. However, in this case, it is necessary to further provide another electromagnetic valve for controlling the control valve having a back-up function. This problematically leads to an increase in cost and an increase in size of the system.
JP-A-3-213773 does not describe such measures in the event of a failure. In addition, JP-A-2006-153104 describes measures against a failure in the electromagnetic valve that controls the shift hydraulic pressure control valve, but does not describe measures against a failure in the shift hydraulic pressure control valve itself.